Grid structure



Inventor- FERRIS Warren fiFerr-Ls y Hts AttOThgg.

GRID STRUCTURE Filed May 29, 1950 July 7, 1936.

Patented July 7, 1936 r 1-:

' "UNITED/STATES" EATENf FFicE New York The present invention relates toelectrondischarge apparatus, more particularly to those devices in whichthe electron stream is intercepted by a plurality of gridmembersi 5 Theusual form of thermionic amplifier com-:

prises-a filament, one or more helical grids. surrounding the filamentand an anode. There may be used two, three, four or more grids dependingon the tube characteristics desiredand theamount of electrostaticcontrol necessary, and for other reasons. The gricl, arrangementgenerally includes a membercalled a control grid whose sole function isto offer a variable control of the electron stream andanother electrodetermed a screen grid which serves to reduce .or neutralize theelectrostatic. capacitynormally existing between the control gridand'the anode and to increase the amplification'of the tube. The turns ofthe screengrid whichis positioned between the control grid andjtheanode, are usually wound closer together, i. ,e. havingless pitch andmore turns per inch of length, than those of the control grid. anditoften happens that theratio of the respective number of turns is two,three or other multiple. .Under an'yf of these conditions, acertaini-numberof turns of. the outer or screen grid will lie between.the adjacent turns of the inner. orjJcontrolfgrid' as viewed from theoutside. of. .the.1 tube looking toward the .filament, and" so besubject to'the electron impact of the .high v,vgalocity electron stream.However, iotheri portions ,of .the screen grid will lie directlyinlfront of the "inner grid wires or directly in bad; of eachcontrolgrid wire considered from the direction of electron traveLandhence will be shielded to a large ergtent from electron impactf It hasbeen found that as the screen grid current is increased, for example, byraising the voltage applied thereto,

40 the screen grid becomes heated along those lengths of wirepositioned'in the unobstructed beam Lof electrons but is relatively coolat" the remaining portions: of. the grid, along'those l ths which. ar ith e tron shadow-of e control r d w s, e cqnd i efii qq i e heatingproduces in the =screen grid alternate lengthsof heated and unheatedvportions which may afiect the capacityneutraliz ing function of themember and cause-I many. other defects such as variationsin the spacecurrent flowing tojgthe anode, fusing nd wa ping'er, th grid s'tructure,also the liberation of :deleteriousjgases; The condition of unevennessof peep ng, infaddition, may reduce the maximunifpower output of thetube which ordinarily wo'uldbe-derived fronitlie use of an additionalspace charge grid because the greater the density of. therelectronstream the" more pronounced is the localized heating. effect. c I

An object of the present invention is to im- 5 provethe 'gridconstruction and arrangement in thermionic devicesto 'the endv that eachof the grid members will be heated uniformly throughout its length. Thisobject is attainedxin brief, by reducing that portion .of the gridsubject to 10 the direct-impact of the.high. velocity electron streamand-to allow the grids to shield one another in the greatestpossibledegree without sacrificing the efficiency of the tube. Accordingto my.invention, the improved arrangement 15 consists in winding thesuccessive grids in opposite directions so that the differences inpitches ofthe respective grid membersbecome additive and the shortestlengthof each gridis presented to the. direct action. of the electronstream. 20 1 The invention will'be better understood when reference islmade to the following description and the accompanying drawing in whichFig. 1 represents an'elevational view partly in'section of a tubeimproved in accordance with my invention;TFig. 2 is aiplan VieW'lOokingdown on the electrode structure; Fig. 3 shows the relation between 'a"pair of-gridsand the 'filament; Fig. 4 151a diagrammatic viewdepictingin rather an exaggerated form by way of comparison the 30 priorlart;method 10f arranging the screen grid andicontrol grid wires in therighthand view, and on-"the -left,: the technique as improved inaccordance iwith my invention is illustrated; Fig. 5riS aview useful inexplaining the invention, and showinggtypical electron paths whichintercept the;grid s, whi le Fig. 6 is a diagrammatic view of a pentodein which my invention finds application, also associated circuits.

- Referring to Fig. 1, numeral I designates an 4 evacuated envelope ofthe tipless type so-called, terminating a combined re-entrant stem andpress: and containinga plurality of electrodes. The electrodeorganization includes. an M shaped filament}; supported at the'bottom bythree 5 stiif wires A, the two outer of which serve asleading-.infconductors. "The upper apexes of the "n "merit; aresupported in the usual manner by n ansof twq uprights" 5 which aresecured in a beadfoffglass (not shown) and supported from 50 the stem 2.Arranged symmetrically about the filament, there are three grids fi, 1,8, also an 'apoeeaas shown moreclearly in Fig. 2,the in- {nerr'iiostgridf6 functioning as the electrostatic control member; the intermediategrid 1 as the 5 screen member and the outermost grid 8 serving to reduceor suppress the secondary emission from the anode. Each grid issupported between respective pairs of vertical rods I secured in thepress 2, one rod per grid except in the case of the outermost grid beingextended through the stem to constitute a leading-in conductor. Theseconductors, in addition to the filamentw leading-in wires, may beconnected to a plurality ofcontact pins ll, molded in a base [2. Theupper ends of the grid rods are held in position by a mica disc or plateI3 bridged between the rods l4 which are secured in the press 2. Theanode; or

plate member 9 takes on a rectangular shape and is provided withrectilinear extensions i5 which embrace the rods I4. The outermost gridor secondary emission suppression member 8 is conveniently formed as acoarse mesh preferably off; nickel, while the inner grids 6 and "I are.wound as flat helixes in order to accommodate their formto the M shapedfilament. The controlngrid 6 is preferably. made of nichromeand. thescreen grid. of. molybdenum. The: present invention is directed moreespecially to the manner of winding the inner grids 6 and I, as will; beexplained hereinafter although it will, be understood. that theprinciples of my inventionapply-equally well to any pair of wound.gridsiin. an electrode organization.

Fig. 6 shows in diagrama. typical. circuit in which the improved tubefin'ds application. In this figure, numeral Iiidesignatesanzinputtransformer connected between they control. grid 8 andfilament 3'. The plate battery or othersource of electromotive force I]serves to energize the plate circuit and the screen. grid circuit inparal: lel while the secondary. emission suppression grid 8 is connectedeffectively to the mid-point of the filament as indicatedby referencecharacter I So; for well understood reasons; this connection isconveniently made within the tube. The output current of the transformermay flow through transformer I8 to a load circuit'. Thecontrol gridcircuit is generally provided-.witlr anegative bias battery l9. 3

From the circuit diagram, it is evident that. a relatively highpotential is: applied tothe screen grid! under operating conditions andit has been found that when the turns on the latter greatly exceedthoseon the control grid; and in amentreme case bear a multiple ora-near multiple ree lation. therewith, certain lengths of the screengrid wiresare excessively heated during opera-.- tion. The reasons forthis effect will be clear from an inspection of Fig. dwhichindicates theelectrostatic fields surrounding the electrodes. In this figure, thefilament 3- isshown as-emi'tting electrons which move toward the plate 9under the. potential gradient established by-the plate battery, whichelectrons are intercepted by the control gridG andthe screen grid. Thewires 6 are surrounded by anegativeelectrostatic field indicated roughlyby the dot and dash lines, a which repel the electrons shown asdottedlines b and converge the same into a highly concen trated mass.sweeping throughthe space between the wires 6. The greatestden'sity of.the ereetron stream occurs mid-way betweenthewires. It is apparent thatthose portions 11) of. the screen grid which'are inthe direct pathof theelectrons, and particularly in the path mid-way between the wires 6;will be subject to greater electron impact thanrscreen grid wires Towhich are in linewith. the control; grid wires, hence; shielded theelectrons.

by the latter. In this figure the secondary emission suppression gridhas been omitted for the sake of clearness although it will beunderstood that such a grid may be employed in combination with theother grids and may also be improved in accordance with the principlesof my invention.

In order to minimize the heating effects of the wires. which areexposedto the full impact of the electron beam, it is apparent that only suchwire as is absolutely necessary should be presented to In accordancewith my invention, this end is accomplished by winding the screen grid;helix in a direction opposite to that of the control grid, as indicatedin the lefthand view of li'i-g. 4... This manner of winding serveseffectively to add the. pitches of the helixes as measured from a commondatum line and to increase the angleatthe point of intersection betweenthe Wiressor that the length of each screen grid wire positioned betweenthe turns of the control grid is reduced tothe minimum. When the wiresare wound in the'same' direction, according to the prior art,thepitches, as measuredfrom the common datum line, are subtractive andthe angle between intersecting wires is very acute. The

advantage winding the grids in opposite di rections is readilyappreciated from the comparison of the twoviews, in Fig. 4. In the priorart arrangement, at the right, the wires 1bof the screen grid areexposed over their entire length between thesupport rods lllto the fullimpact of the-electron stream and hence will-seriously heat, while theadjacent wires-1c are'fo'r the most part shielded by the controlgrid'wire 6 and will not heat to any appreciable extent. However; whenthe wires cross inopposite directions, asindicated in the lefthandview,the effective length of screen grid wire inthe space between theturns of the control gridis materially shortenedand the heatingv efiectlreduced in like: degree. Thus; in the improved construction the lengthof wire "1 whichdiagonally spanseachpair of control wires ET is; muchless than the distance between the inner supportrod's Thence less thanthe length of screen gridwire of the prior art arrangementwhichwoul'dnnormally be exposed to the full impact oii theelectrons. It.will also be noted that thewires. 12:, which in the prior artconstruction werecompletely shielded by the control grid, are nowexposed to the electrons-in substantially the same amount astheiwiresJb, hence, subjected, to substantially thesame heatingeifect'.

While-my invention. has been explained with particular reference to apair of grids having turns-in a. one-to-tworelation it; is evident thattheimprovement.applies-to all turn ratios and more.especially; to thosecases inwhich the number of;turns on; the; screen grid isslightly'less-or greater; than aamultiple of the number on thecontroligri'dz one type of, tube improved in accordance with: myinventionv the control grid had I55 turnsper; inch and the screen grid34 turns-per-inch and itwas foundthat the screen grid washeated'uniformly over'its' entire length. Ontheother handj whenxthegrids were wound in they same" direction according to the teachings ofthe prior art, the wire became heated in lengths approximately /3 7apart. v Thev advantages ofieredlby my invention are seeuredwithout.'detracti'nginany. substantial, degree; from. the control. exercisedbythe various grid; members and therefor-ado not affect the electrical;characteristics ofthe tube.

What I claim as new and desire to secure by tional grid comprising awire wound in a. plu- Letters Patent of the United States is: rality ofturns in the opposite direction.

1. An electron discharge device comprising a 3. In an electron dischargedevice, the combicathode, an anode, a plurality of helically woundnation of two grids each comprising a wire wound 5 grids positionedtherebetween, a pair of said in a plurality of turns in one direction,and an 5 grids being wound on the same axis and in opadditional gridbetween the two first mentioned posite directions. grids comprising awire wound in a plurality of 2. In an electron discharge device, thecombiturns in the opposite direction. nation of a grid comprising a wirewound in a 10 plurality of turns in one direction, and an addi- WARRENR. FERRIS. 10

